Save the trees

Duncan Denley of desert INK argues that landscape architects should focus on what’s below the ground when it comes to hosting large trees in our public realm

I’d like to start on this topic by retelling a story which a friend recently passed on to me. Once upon a time, a small, happy tree growing in a forest was beloved by the humans settled there. The tree’s roots reached as far they liked into the moist, cool soil. As time went on, the humans’ paved over the soil and removed the tree. But the humans missed its bushy canopy and sturdy branches, so they dug a small pit in the paving, filled it with soil and put in a tree. The new tree’s happiness was short-lived, as its roots quickly filled the hole and used up all of the nutrients. The tree became weaker and weaker and one day, with only small, weak roots to hold it in place, it was toppled by the wind.

We’ve all experienced it; the client wants a lush green landscape, with bushy shade trees lining their beautiful new streetscape. An instant landscape which adds value to the neighbourhood, giving a feeling of prosperity and proximity to nature. Indeed there are countless studies which prove that tree-lined streets increase property values massively. The City of New York has started to calculate the annual benefit of its 600,000 large street trees, citing this at 122m USD, more than five times the cost of maintaining them.

But here’s the thing – we all know this instinctively. People love to be around big trees. It makes them feel good. I mention big trees specifically, and that’s because only big trees give that all-important feeling of enclosure. Humans are programmed to enjoy the feeling of enclosure and protection that big trees offer, so although a 3m tree is nice to look at, it doesn’t connect with our sense of wellbeing to the same extent.

So we are agreed. We need big trees in our cities, but we also need paved surfaces for vehicles. What is the answer?

The fundamental problem of the tree pit method highlighted in the story above is that the tree’s roots are unable to penetrate the highly compacted, poor quality earth below paved areas. A low-cost solution to this problem is structural soils to the areas surrounding the tree pit. This involves mixing sweet soil with clay (often sourced locally at dams) and a very angular aggregate mix and applying this below the paving. Structural soils can be compacted to 95%, allowing for paving to be installed above it, while the aggregate allows pockets of soil to exist throughout. Such simple systems allow the tree’s roots access to a much greater area to collect moisture and nutrients, and this solution is tried and tested in the Middle East. There are two downsides, however: firstly, it costs more than the traditional method of planting trees in small tree pits, and secondly, only about 20% of structural soil’s volume is soil supporting root development.

For clients with deeper pockets and a good understanding of the commercial value of large trees, there is something much better. There are various structural cells available in the marketplace which are effectively load-bearing, plastic modules which are filled with soil. These systems are placed below the surrounding paving, ideally linking a succession of tree pits. This creates an infinitely larger area of uncompacted soil through which tree roots can spread, while allowing paving and roads to be installed above. Such systems are widely used over much of the developed world to great success.

The downside? If we consider that a standard tree, pit and stake might cost about AED 1,000, then about AED 4,500 is required for the same tree, pit and stakes with an additional 30m3 of structural soil surrounding it. To add 30m3 of structural cells and soil around your tree pit will cost an additional AED 8,000.

So desert INK are considering how to find the required funds to install structural cells in our streetscape designs going forward, and we urge other landscape architects to do the same. We ask ourselves, which has the greater positive impact on our streets: large, wonderful trees, or expensive luxury materials, fixtures and fittings?